The Sextant

The major
problem with back-sight
instruments was that it was difficult if
not impossible to sight the moon, the
planets or the stars. Thus, toward the
end of the 1600's and into the 1700's,
the more inventive instrument makers
were shifting their focus to optical
systems based on mirrors and prisms that
could be used to observe the nighttime
celestial bodies.

The critical development was made
independently and almost simultaneously
by John Hadley in England and by Thomas
Godfrey, a Philadelphia glazier, about
1731. The fundamental idea is to use of
two mirrors to make a doubly reflecting
instrument—the forerunner of the modern
sextant.

How does such an instrument work? Hold the instrument
vertically and point it toward the
celestial body. Sight the horizon
through an unsilvered portion of the
horizon mirror. Adjust the index arm
until the image of the sun or star,
which has been reflected first by the
index mirror and second by the silvered
portion of the horizon mirror, appears
to rest on the horizon. The altitude of
the heavenly body can be read from the
scale on the arc of the instrument’s
frame.

Hadley's first doubly reflecting
octants were made from solid sheets of
brass. They were heavy and had a lot of
wind resistance. Lighter wooden
instruments that could be made larger,
with scales easier to divide accurately
and with less wind resistance quickly
replaced them.

Early
Hadley octant. This mahogany
octant was made about 1760 by
the famous London maker, George
Adams.

Hadley' octant of 1731 was a major
advancement over all previous designs
and is still the basic design of the
modern sextant. It was truly a "point
and shoot" device. The observer looked
at one place - the straight line of the
horizon sighted through the horizon
glass alongside the reflected image of
the star. The sight was easy to align
because the horizon and the star seemed
to move together as the ship pitched and
rolled.

We have seen how navigators could
find their latitude for many centuries
but ships, crews and valuable cargo were
lost in shipwrecks because it was
impossible to determine longitude.
Throughout the seventeenth century and
well into the eighteenth century, there
was an ongoing press to develop
techniques for determining
longitude.
The missing element was a way to measure
time accurately. The clock makers were
busy inventing ingenious mechanical
devices while the astronomers were
promoting a celestial method called
"lunar distances". Think of the moon as
the hand of a clock moving across a
clock face represented by the other
celestial bodies. Early in the 18th
century, the astronomers had developed a
method for predicting the angular
distance between the moon and the sun,
the planets or selected stars. Using
this technique, the navigator at sea
could measure the angle between the moon
and a celestial body, calculate the time
at which the moon and the celestial body
would be precisely at that angular
distance and then compare the ship’s
chronometer to the time back at the
national observatory. Knowing the
correct time, the navigator could now
determine longitude. When the sun passes
through the meridian here at Coimbra,
the local solar time is 1200 noon and at
that instant it is 1233 PM Greenwich
Mean Time. Remembering that 15 degrees
of longitude is equivalent to one hour
of time gives us the longitude of 8
degrees, 15 minutes West of Greenwich.
The lunar distance method of telling
time was still being used into the early
1900’s when it was replaced by time by
radio telegraph.

An octant measures angles up to 90
degrees and is ideally suited for
observations of celestial bodies above
the horizon. But greater angle range is
needed for lunar distance observations.
It was a simple matter to enlarge
Hadley's octant, an eighth of a circle,
to the sextant, a sixth of a circle,
that could measure up to 120 degrees.

An
early sextant by John Bird.
The first sextant was produced
by John Bird in 1759. This is a
very early example of his work
now in the Nederlands
Scheepvaart Museum in Amsterdam.
The frame is mahogany with an
ivory scale. It is so large and
heavy that it needed a support
that fitted into a socket on the
observers belt.

A brass
sextant by Dollond. Here’s a
fine brass sextant from the
early nineteenth century by the
master London instrument maker
John Dollond.

In the first
half of the eighteenth century there was
a trend back to wooden frame octants and
sextants to produce lighter instruments
compared to those made of brass.

Ebony
sextant. A very handsome
example by H. Limbach of Hull of
a sextant with an ebony frame.
Ebony was used because of the
dense wood's resistance to
humidity. The scale and vernier
were divided on ivory, or should
we now say bone. The design was
not successful because the wood
tended to split over the long
arc of a sextant.

Examples of sextant frame
designs. A sample of
variations in frame design. The
challenge was to produce sextant
frames that were light weight,
low wind resistance and with a
minimum change is dimensions
with changes in temperature. As
you can see, some of them are
quite esthetically pleasing.

Probably the finest 18th
century instrument maker was the
Englishman Jesse Ramsden. His specialty
was accurate scale division.Here’s a small brass sextant that
Ramsden made shortly before his death in
1800. Ramsden's major achievement was to
invent a highly accurate "dividing
engine"—the apparatus used to divide the
scale into degrees and fractions of
degrees. His design was considered so
ingenious that the British Board of
Longitude awarded Ramsden a prize of 615
pounds—in 18th century terms,
a small fortune. His "dividing engine"
now resides in the Smithsonian
Institution in Washington.

Ramsden pentant.
To be correct, the instrument
should be called a pentant, a
fifth of a circle, rather than a
sextant. This jewel is only 4
1/2 inches radius. The scale is
divided on silver from minus 5
degrees to 155 degrees with each
degree further divided in three
to 20 arc minutes. As you can
see, the scale is beveled at 45
degrees. Why set the scale at an
angle to the frame - perhaps
just to show that he could do
it!

The development of more precise scale
division was a milestone in instrument
development. Certainly, it permitted
more accurate observations but it also
permitted smaller, lighter, more easily
handled instruments. The sextant you see
here is my all-time favorite.

Modern
sextant, 1988

The standard of excellence for post
World War II sextants was established by
the C. Plath firm in Germany. Here's an
example from 1988. Among its attachments
are an unsilvered horizon glass that
lets the observer see the full horizon
as a straight line across the round
horizon glass; an astigmatizer lens that
distorts the image of a star into a
straight line for precision alignment
with the line of the horizon; a
quick-release drum micrometer that reads
to one-tenth of an arc minute. There’s
also a battery-supplied lighting system
for the drum micrometer and the bubble
artificial horizon attachment. This
attachment and a monocular telescope
complete the kit. But, for all the fancy
modern refinements, the optical system
is exactly what John Hadley proposed in
1731.

The problem of finding your location
when you can’t see the horizon to take a
sun or star sight has challenged
explorers, map makers and navigators for
hundreds of years. Early in the 1730s
instrument makers began developing
artificial horizons for use with
quadrants. Of course, the explorers and
mapmakers working inland could not use
the horizontal line to the natural
horizon of the sea and so they needed an
artificial horizon to establish a line
of reference for measuring the altitude
of celestial bodies.

Mercury
artificial horizon. A very
elegant three-piece explorer and
mapmaker's kit by Carey of Pall
Mall, London from 1880. The
instrument is a pentant, a fifth
of a circle capable of measuring
angles up to 170 degrees;
mounted on a collapsible
aluminum stand. Around the base
you can see the parts of the
mercury bath artificial horizon.
Mercury was poured from the iron
bottle into the trough to form a
shiny horizontal surface to
catch the reflection of the
celestial body. The triangular
glass tent was placed over the
trough to keep the wind from
disturbing the surface.

A
mercury artificial horizon in use.
Here you see the famous American
explorer, John Charles Freemont,
using a sextant and mercury
artificial horizon to find his
position during his expedition
of 1842 to map the Oregon Trail.
The sextant had to be pointed
downward to view the reflection
of the celestial body on the
surface of the mercury pool
through the clear portion of the
horizon glass while
simultaneously adjusting the
index system to bring the image
reflected by the two mirrors
alongside. The mercury
artificial horizon was popular
with explorers for more than a
century but it was hard to use
on shipboard with a rolling
deck.

A little earlier, we were talking
about the explorers' and mapmakers' need
for an artificial horizon when they
couldn't see the natural horizon. Well,
there are two classes of modern
navigators who absolutely need an
artificial horizon - the aviators and
the submariners. Aviators find the
natural horizon so far below them that
it is useless and furthermore, they are
frequently flying above the clouds.
Conversely, even on the surface,
submariners are so low in the water that
a sight to the horizon is unreliable. In
fact, it is the unique needs of the
aviator that has driven sextant
innovation throughout the twentieth
century.

For a while, balloonists of the late
nineteenth century tried to use
conventional sea-going sextants but
their need for artificial horizon
instruments soon became apparent.

Balloon
sextants. The optical
concept of these instruments is
to the reflect the image of a
bubble from a small spirit-level
into the line of sight so that
the bubble and the celestial
body can be viewed
simultaneously. The one at the
top, from 1880, is derived from
an instrument invented by
Captain Abney many years earlier
for use in chart making. The
instrument in the middle is by
Cary of London, 1900, and the
one at the bottom is one of
their later models with an
electrical lighting system from
1910 - just about the time of
the Wright brother's first
powered flight.

The rapid development of
heavier-than-air craft during World War
I lead to airplanes with increasing
range and greater need for accurate
navigation instruments and techniques,
all depending on artificial horizons.

Gyroscopic aircraft sextant.
An early 1920's gyroscope
sextant by a Parisian company
with the descriptive name of
La Precision Moderne. A
spinning mirror, mounted on the
top of an air driven gyroscope
reflects an image of the
celestial body into the line of
sight, much as with the
old-fashioned mercury artificial
horizon.

One of the most important pioneering
trans-Atlantic flights was by the famous
Portuguese aviators, Sacadura Cabral,
pilot, and Admiral Gago Coutinho,
navigator, in 1919. They flew 11 and one
half hours from Cape Verde Islands to
Rio de Janeiro carrying an artificial
horizon sextant designed by Admiral
Coutinho.

The
System Gago Coutinho. The
design was based on two spirit
level tubes – one to keep the
sextant horizontal and the other
to keep the sextant vertical.
The sextant proved itself again
in a flight from Lisbon to Rio
de Janeiro in 1927 with Captain
Jorge Castilho as navigator.

The Portuguese Navy, who had rights
to the development, contracted with the
prestigious German firm of C. Plath for
production. In 1929 Captain Wittenman
navigated the Graf Zeppelin around the
world using a Coutinho sextant. With
this spectacular record, the design was
the hit of the 1930 Berlin Air Show. It
was used by many of the major airlines
of the world throughout the 1930’s. In
addition to an artificial horizon,
aircraft sextants needed a device to
average the values of six or eight
sights taken in succession to average
out the small errors in aligning the
sight and to compensate for the rapid
movement of the aircraft. Here are some
prewar examples.

Early
bubble sextants with averagers

WWII
Aircraft sextants

Of course, World War II was a
powerful influence that produced an
explosion of designs and a number of
U.S. instrument makers Fairchild, Link,
Pioneer and Agfa-Ansco made important
improvements. C. Plath in Germany and
Tamaya in Japan supplied the Axis

There has been very little evolution
of hand-held celestial navigation
instruments since the end of World War
II. Faster flying aircraft lead to the
development of periscope instruments
that minimized wind resistance but Radio
Direction Finding and then inertial
guidance became the standard for
aircraft navigation and celestial was no
longer needed.

The early space flights used an
especially designed sextant. In the
remoteness of space there is no such
thing as "horizontal" or "vertical".
Instead, the instrument was designed to
measure the angle between the edges of
the earth or the angle between celestial
bodies to determine the space craft's
position in space. But again, electronic
techniques for positioning in space
became the standard.